High-temperature Plasma Jet Research Articles

Fast deposition of diamond-like and diamond films using a plasmatron with interelectrode inserts

Data on the deposition of thin wear-resistant diamond-like films (DLFs) and diamond films (DFs) are reported. Films are deposited using the chemical vapor deposition method with the application of a supersonic high-temperature plasma jet generated by a high-enthalpy plasmatron with a sectionalized interelectrode insert and a cold gas curtain of discharge chamber walls. The deposition rate for DLFs and DFs is as high as 30–50 μm/h. The best results (the absence of the graphite phase in a film) are obtained at the highest degree of ionization of the supersonic plasma jet.

Heat and Mass Transfer Within an Evaporating Solution Droplet in a Plasma Jet

Solution precursors have been injected into the plasma gases to produce finely structured ceramic coatings with nano- and sub-micrometric features. The trajectory history and heat and mass transfer within individual solution droplets play a very important role in determining the coating microstructure. A mathematical model is developed to analyse the thermal behavior of individual precursor droplets travelling in the high temperature plasma jet. This model involves the motion and evaporation of the precursor droplet in a DC plasma jet and the heat and mass transfer within the evaporating droplet. The influence of Stefan flow, as well as the variable thermo-physical properties of the solution and the plasma gas, is considered. The internal circulation due to the relative velocity between the droplet and the plasma jet, which may be approximated by the Hill vortex, is considered as well. The trajectory, temporal droplet surface temperature, and radius variation are predicted. The temporal temperature and concentration distributions within the evaporating droplet are presented for different injection parameters.

Ignition in a supersonic flow by a plasma jet of mixed feedstock including CH 4

Ignition tests of hydrocarbon fuels in a supersonic airflow by plasma jet (PJ) torches of mixed feedstock, including methane (CH 4), such as N 2/CH 4 and N 2/CH 4/O 2 mixtures, were conducted. The Mach number of the airflow was 2.0, and the total temperature and total pressure of the main flow were those of room conditions. The wall pressure increase due to combustion of hydrocarbon fuels for the N 2/CH 4 PJ exceeded those of pure O 2 and N 2 PJs at high electric power input. Equilibrium calculations showed that the main species in high-temperature PJ, aside from N 2, were H 2, H, and HCN. Considering the slight impact of the HCN species on ignition delay time, the combustion enhancement by the N 2/CH 4 PJ was caused primarily by the existence of a large amount of H and H 2 dissociated from CH 4 molecules in the PJ. Moreover, the addition of O 2 to the N 2/CH 4 feedstock further enhanced the combustion and stability of the N 2/CH 4 PJ. The existence of O 2 increased the temperature and the number of H radicals in the PJ exhaust.

Review of Modeling of Liquid Precursor Droplets and Particles Injected into Plasmas and High-Velocity Oxy-Fuel (HVOF) Flame Jets for Thermal Spray Deposition Applications

This article presents a review of the current state-of-the-art in modeling of liquid chemical precursor droplets and particles injected into high-temperature jets in the form of DC-arc plasmas and high-velocity oxy-fuel flames to form coatings. Conventional thermal spray processes have typically utilized powders that are melted and deposited as a coating on hardware surfaces. However, production of coatings utilizing liquid precursors has emerged in the last decade as a viable alternative to powder deposition. Use of liquid precursors has advantages over powder in terms of their relative ease of feeding and tailoring of chemical compositions. In this article, we review the modeling approaches to injection of liquid precursors and particles into plasmas and high-velocity oxy-fuel flames. Modeling approaches for the high-temperature DC-arc plasma and oxy-fuel flame jets are first reviewed. This is followed by the liquid spray and droplet level models of the liquid precursors injected into these high-temperature jets. The various knowledge gaps in detailed modeling are identified and possible research directions are suggested in certain areas.

X-ray jets and magnetic flux emergence in the Sun

AbstractMagnetized plasma is emerging continually from the solar interior into the atmosphere. Magnetic flux emergence events and their consequences in the solar atmosphere are being observed with high space, time and spectral resolution by a large number of space missions in operation at present (e.g. SOHO, Hinode, Stereo, Rhessi). The collision of an emerging and a preexisting magnetic flux system in the solar atmosphere leads to the formation of current sheets and to field line reconnection. Reconnection under solar coronal conditions is an energetic event; for the field strengths, densities and speeds involved in the collision of emerging flux systems, the reconnection outflows lead to launching of high-speed (hundreds of km/s), high-temperature (107 K) plasma jets. Such jets are being observed with the X-Ray and EUV detectors of ongoing satellite missions. On the other hand, the spectacular increase in computational power in recent years permits to carry out three-dimensional numerical experiments of the time evolution of flux emerging systems and the launching of jets with a remarkable degree of detail.In this review, observation and modeling of the solar X-Ray jets are discussed. A two-decade long computational effort to model the magnetic flux emergence events by different teams has led to numerical experiments which explain, even quantitatively, many of the observed features of the X-ray jets. The review points out that, although alternative mechanisms must be considered, flux emergence is a prime candidate to explain the launching of the solar jets.

Numerical simulation of temperature and velocity fields in plasma spray

Based on the turbulence jet model, with respect to Ar-He mixture plasma gas injecting to ambient atmosphere, the temperature filed and velocity field under typical working conditions were investigated. Given the conditions of I=900 A, F Ar = 1.98 m3/h, F He = 0.85 m3/h, it is found that both the temperature and the velocity undergo a plateau region near the nozzle exit (0–10 mm) at the very first stage, then decrease abruptly from initial 13 543 K and 778.2 m/s to 4 000 K and 260.0 m/s, and finally decrease slowly again. Meanwhile, the radial temperature and radial velocity change relatively slow. The inner mechanism for such phenomena is due to the complex violent interaction between the high-temperature and high-velocity turbulent plasma jet and the ambient atmosphere. Compared with traditional methods, the initial working conditions can be directly related to the temperature and velocity fields of the plasma jet by deriving basic boundary conditions.

Particle Injection in Direct Current Air Plasma Spray: Salient Observations and Optimization Strategies

External injection of high-melting point low thermal conductivity ceramics orthogonal to a typical direct current thermal plasma jet plays a vital role in determining the in-flight state of the particles and the process downstream. The interactions between low density ceramic particles and high temperature plasma jet is quite complex, which influences the spray process and associated deposition. Detailed in-flight particle diagnostics as well as spray stream visualization have significantly enhanced our capability to diagnose and control the process. In this paper we present some salient observations on the role of key variables on particle injection. A number of experiments were conducted using a 7MB torch (Sulzer Metco, Westbury, NY) with both Ar–H2 and N2–H2 plasma gases, where the carrier gas flow to inject Yttria Stabilized Zirconia (YSZ) was varied systematically and the resulting in-flight particle state was captured using an array of particle and spray stream sensors arranged in a 3D set-up. A notable observation is the existence of a “sweet-spot” in the plasma jet where the particle temperatures and velocities achieved a maximum. This sweet-spot can be characterized by the plume position (location of centroid of the spray stream) rather than carrier gas flow rate and is independent of primary gas flows and other process/material conditions. This result suggests a possible approach to optimize particle injection independent of plasma-forming-torch-parameters. Controlling particle injection at this sweet-spot has shown to benefit the overall process efficiency (in terms of melting) and process reliability (both in-flight measurement and coating build-up) with concomitant application benefits.

Temperature Compensation and Improved Ballistic Performance in a Solid-Propellant Electrothermal-Chemical (SPETC) 40-mm Gun

A solid-propellant electrothermal-chemical (SPETC) 40-mm gun has been designed, constructed, and tested in the indoor firing facilities of the Soreq Propulsion Division Laboratory. An external injector device converts the electric energy stored in the capacitors of the pulse forming network (PFN) into a high-temperature plasma jet that penetrates the gun cartridge and boosts the whole ballistic process. However, unlike large-caliber SPETC systems, in which electric energy is limited to ignition purposes, the 40-mm SPETC gun is a genuine hybrid gun with almost equal electric and chemical contributions. There is experimental evidence that this unique feature induces a very peculiar initial propellant temperature compensation mechanism. It seems that when a significant part of the propelling energy comes from the plasma, i.e., electric energy is not only predominant at the ignition stage of the firing but also later on, then the temperature sensitivity of the propellant tends to vanish. A simple theoretical model supports the experimental findings. The large amount of electric energy is also responsible for a recorded ballistic improvement of 15% in the projectile muzzle kinetic energy. Calibrated simulations show that an optimal tailoring of the power pulse shape and suitable propellant grain geometry should further increase by 10% the muzzle kinetic energy. These modifications are in progress and results should be soon available

Emission spectroscopy of highly ionized high-temperature plasma jets

This paper deals with advanced studies on the optical emission spectroscopy of atmospheric pressure highly ionized high-temperature argon and nitrogen plasma jets generated by a powerful arc plasmatron. The emission spectra are taken in the 200–1000 nm range with a spectral resolution of ∼0.01–0.02 nm. The exposure times are 6 × 10−6–2 × 10−2 s, the spatial resolution is 0.02–0.03 mm. The recorded jet spectra are abundant in spectral lines originating from different ionization stages. In nitrogen plasmas, tens of vibronic bands are also observed. To interpret and process these spectra such that plasma characteristics can be derived, a purpose-developed automated processing system is applied. The use of a CCD camera at the spectrograph output allows a simultaneous recording of the spectral and chord intensity distributions of spectral lines, which can yet belong to the overlapped spectra of the first and second orders of interference. The modern optical diagnostic means and methods used permit the determination of spatial distributions of electron number densities and temperatures and evaluation of rotational temperatures. The radial profiles of the irradiating plasma components can also be obtained. Special attention is given to the method of deriving rotational temperatures using vibronic bands with an incompletely identified rotational structure.

A plasmatron with interelectrode inserts for depositing diamond and diamond-like films

A plasmatron with a sectionalized interelectrode insert, intended for heating argon up to a temperature of 10 000 to 11 000 K, is described. Investigations have shown that the volt-ampere characteristics of an arc in an argon flow are ascending in the operating range of the plasmatron parameters. The thermal efficiency is as high as 0.9. The plasmatron is used in the chemical gas-phase deposition of thin wear-resistant diamond-like films with the application of a high-temperature supersonic plasma jet.

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

Thermal spray offers a variety of sub-sets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni–5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress–strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni–5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms—splat quenching, peening and thermal mismatch—are discussed with respect to process parameters and material properties.

Entrainment in high-velocity, high-temperature plasma jets. Part I: experimental results

The development of a high-velocity, high-temperature argon plasma jet issuing into air has been investigated. In particular the entrainment of the surrounding air, its effect on the temperature and velocity profiles and the subsequent mixing and dissociation of oxygen has been examined in detail. The total concentration of oxygen and the velocity and temperature profiles in the jet were obtained from an enthalpy probe. High-resolution Thomson scattering provided an independent measure of plasma velocity and temperature, validating enthalpy probe measurements and providing non-intrusive measurements near the nozzle exit. The concentration of atomic oxygen was obtained from 2-photon laser induced fluorescence. Molecular oxygen concentration and temperature was obtained from coherent anti-Stokes Raman spectroscopy. It was found that both the incompleteness of mixing at the molecular scale and the rate of oxygen dissociation and recombination affects jet behavior.

Entrainment in high-velocity, high-temperature plasma jets.: Part II: computational results and comparison to experiment

The development of a turbulent high-velocity, high-temperature argon plasma jet issuing into air has been computationally modeled using the computer program LAVA. LAVA is a comprehensive computational software program developed for flowing thermal plasmas in the absence of electromagnetic fields, with particular emphasis on plasma jets. The plasma is represented as a multicomponent chemically reacting ideal gas with temperature-dependent thermodynamic and transport properties. The plasma flow is governed by the complete transient, compressible Navier–Stokes equations in two-dimensions in the current simulation. Turbulence is represented by the k– ε model. Neutrals, ions and electrons are considered as separate components of species of the mixture. General kinetic and equilibrium chemistry algorithms compute ionization, dissociation, recombination and other chemical reactions. Computational results and extensive comparisons with experimental data are presented. In particular the influence of inflow boundary conditions and the ability of the k– ε turbulence model to describe entrainment with chemistry are examined.

Application feasibility of high-performance-type plasma jet device to various material processes

A newly designed high-performance-type plasma jet device for processing has been developed. This device can generate a high temperature and clean plasma jet and inject the process material into the center of the arc column. In order to demonstrate the application feasibility of this device to thermal processing, the effects of nozzle diameter and material loading on the characteristics of the arc and plasma jet were studied. On increasing the nozzle diameter, the jet power decreases by 0.5–1 kW but the stable operating region expands. In addition, this device operates stably with material loading. It is confirmed that the new type of device proposed would be very useful for various material processes.

Optical measurements of plasma velocity and temperature in a low-rate, low-power LPPS system

Low-pressure plasma spraying (LPPS) is a promising method of applying coatings to surfaces for thermal, chemical, and mechanical protection. The deposition of coatings with superior properties requires an improved understanding of this complex spray process, including the detailed understanding of the time–temperature and chemical history to which an injected particle is exposed during its travel through the high-temperature and high-velocity plasma jet. In this paper, we report on measurements of the temperatures and velocities within the plasma in a low-pressure argon–hydrogen arcjet spray plasma being developed for mesoscale manufacturing of mechanical components. The measurements reported here make use of laser absorption, laser-induced fluorescence, and optical emission spectroscopy of the H α Balmer transition in atomic hydrogen. Both the absorption and fluorescence methods take advantage of the well-known spectral line broadening of atomic hydrogen, to extract the arcjet temperature and electron density. Jet axial velocities are measured via the time of flight, as detected by optical emission, of convected disturbances caused by intrinsic fluctuations in the arc voltage. Measurements made for various anode (nozzle) geometries indicate that relatively low (∼1 km s −1 mm −1) velocity gradients in the axial direction could be achieved with nozzles that include an expanding section of 10° half-angles. The measured temperatures and velocities suggest the presence of complex gas-dynamic shock structures in the arcjet plume, depending on the particular nozzle length and expansion section.

Influence of a Counterflow Plasma Jet on Supersonic Blunt-Body Pressures

Aerodynamic augmentation in the presence of a thin high-temperature onboard plasma jet directed upstream of a slightly blunted cone was studied experimentally and numerically. The flow around a truncated cone cylinder at zero incidence was considered for Mach numbers M∞ = 2.0, 2.5, and 4.0. For the first time, computationally validated experimental pressure distributions over the model surface in the presence of the plasma jet were obtained. As in the conventional (nonplasma) counterflow jet, two stable operational regimes of the plasma jet were found. These were a short penetration mode and a long penetration mode (LPM) aerospike into the opposing supersonic freestream. The greatest drag reduction occurred in the moderate LPM regime. LPM strong overblowing reduces the benefits. The experimental pressure results were approximately validated against an Euler computational fluid dynamics simulation, modeling a perfect gas hot jet, counterflowing against a perfect gas supersonic freestream. Plasma effects such as electron pressure, radiation, electric field interactions, Joule heating, and induced vorticity, streamers, and plasmoids have been identified that, if accounted for, may improve the comparison. Procedures for the use of these experimental results have been outlined as a baseline that will be useful in separating fluid dynamic/thermal effects from plasma processes in understanding the physics of onboard plasma jets for aerodynamic augmentation.

Plasma electrode-type plasma spray gun—effect of powder loading on the behavior of plasma jet

A newly designed plasma electrode-type plasma spray gun, which can generate a high temperature and clean plasma jet and inject various materials into the center of the arc column, has been developed. In order to demonstrate the application feasibility of this new-type plasma spray gun for thermal plasma processing, the effects of powder loading on the stability and temperature of the plasma jet and on the electrode spot were studied. It was clarified that with powder loading, the plasma jet was very stable and not contaminated by electrode materials, i.e. clean, and the extent of the jet temperature decrease was a little, i.e. about 1000 K.

Modeling and experimental observation of evaporation from oxidizing molybdenum particles entrained in a thermal plasma jet

A model describing the in-flight evaporation of particles injected into a high-temperature plasma jet issuing into surrounding air has been developed and incorporated into an earlier model that includes a detailed description of particle heating and melting. In addition to physical evaporation controlled by vapor diffusion and heat transfer, the evaporation due to the production of volatile oxides on the particle surface is also modeled. The effect of evaporation-induced mass transfer on heat flux to the particle surface is taken into consideration along with the effects of variable plasma properties and the modification of heat and momentum transfer due to non-continuum effects experienced under plasma conditions. Computational results on molybdenum particles in an argon–hydrogen DC plasma spray system are developed and are qualitatively compared with experimental results.

Boron nitride nanotubes, webs, and coexisting amorphous phase formed by the plasma jet method

Boron nitride (BN) nanotubes were prepared by a high-temperature plasma jet method. While the BN nanotubes were found in quantity on the surface of a BN sintered disk, amorphous BN webs were found dominant in the water-cooled collector. These structures were studied on the basis of transmission electron microscopy observations. One of them was composed of 21 layers grown coaxially with a regular spacing of 0.34 nm and their tip ends were terminated with parallel bases of the coaxial cylindrical shapes. The diameter of the innermost tube was 0.6 nm and this dimension is in fairly good agreement with the diameters of the smallest BN fullerenes.

Scheme for stationary continuous-wave vacuum- and extreme-ultraviolet lasing based on a plasma jet

A scheme to achieve stationary continuous-wave (cw) lasing in the vacuum- and extreme-ultraviolet spectral region is proposed. It is based on recombination pumping in a freely expanding plasma produced in a high pressure, high temperature plasma jet. Stationary cw-lasing at 1640 Å or 729 Å can be expected by inverting the He II (2←3) or Li III (2←3) Balmer-α transition following three-body recombination of bare He2+ or Li3+ ions, respectively. The expected gain coefficients are calculated using a time dependent collisional-radiative model and estimates of the required pumping power as well as considerations on the technical feasibility of the concept are presented.